An aerospace structure and method for its construction composed of two elements having mutually opposed mating surfaces, at least one of which is irregular. The mating surfaces of the elements are fastened and compressed against a noncompressible cast-in-place void filler having complimentary shape to the two mating surfaces. The void filler is prepared by admixing a polymerizing matrix selected from polyesters, epoxides, polyamides, polyimides, polycarbonates, polyvinyl alcohols and organosilicone polymers and a space filling material selected from phenolic microspheres, carbon microspheres, metal coated microspheres, chopped fiberglass, alumina, fumed silica and mixtures thereof.